Hydroxide slurries are aqueous suspensions of solid hydrated oxides of primarily magnesium, as Mg(OH)2, calcium Ca(OH)2 and mixtures thereof, in water. They are widely used in many industrial processes, an example of which is the treatment of water to raise the pH and eliminate odors, particularly for sewerage treatment, and to precipitate heavy metals. These slurries are increasingly replacing sodium hydroxide because of their inherent properties.
For slurries with a high pH of about 12.0, a hydrated lime slurry Ca(OH)2 or dolime slurry Ca(OH)2.Mg(OH)2 is used, whereas for slurries with a pH of about 10.4 when diluted in water, a magnesium hydroxide slurry Mg(OH)2 or semidolime Mg(OH)2.CaCO3 slurry is used. For sewerage treatment, magnesium hydroxide slurry or semidolime slurry is preferred because any excess magnesium hydroxide entering digesters does not kill the bacteria, whereas overdosing of sodium hydroxide or hydrated lime slurry can destroy the bacteria and close down the digestion process. In heavy metal removal, the pH of the magnesium hydroxide or semidolime slurry is in the desirable range where amphoteric hydroxides of toxic metals precipitate, whereas at the pH of sodium hydroxide or hydrated lime, the initial precipitates re-dissolve.
Hydrated lime slurries are produced using lime, CaO, from a kiln, and the hydrators for that process are known in the art. Generally, the lime is sufficiently reactive that the slurry is formed quickly, in typically less than sixty minutes from granules of the order of 1 mm. The process may include grinding the lime, and generally the process requires the addition of dispersants to provide the required stability and to assist the slurry production process. Many of these dispersants are destroyed at high temperature, so that a lime hydrator is generally cooled.
Magnesium hydroxide slurries are produced from either precipitated magnesium hydroxide or from hydrating magnesium oxide, which has been generally produced from the calcination of the mineral magnesite.
Preferred properties of hydroxide slurries may include:                1. The slurry should contain particles that can react quickly when the slurry is, for example, mixed with waste water. Thus, a slurry may be composed of small particles with a median size of 5 μm and a low surface area about 4 m2/gm (as measured by gas absorption methods of the dried powder), or a median size of 20 μm and a high surface area about 2.0 m2/gm. A wide, or multiple peaked, particle size distribution, with a sharp cut-off, is preferable to promote the slurry stability. Such a particle size distribution d90 may be less than 100 microns.        2. The chemistry of the surfaces of the particles in the slurry are strongly dependent on the surface area of the oxide particles that are hydrated to form the slurry, and the range of applications of the slurry depend on these surface chemical properties. Thus, a slurry produced from oxide particles that have a surface area on the order of 150 m2/gm or higher have markedly different exterior chemical surface properties than a slurry produced from conventional oxide with a surface area of 20 m2/gm. Without being bound by theory, oxide particles having a high surface area have larger amounts of energetic chemical defects, such as superoxides and peroxides, at the crystal grain boundaries, and these species largely survive hydration to confer on the slurry particle surfaces a different degree of surface chemistry that correlates with the surface area of the initial oxide used to form the slurry.        3. The percentage of solids by weight is at least 50%, and preferably 55-65%. The higher the solids content, then the less water is required to be shipped. However, the higher limit is generally the result of the requirements that the slurry has a low resistance to thinning and, thus, a low apparent viscosity at low shear rates. If the solids fraction is too high, a gel tends to form that has a high resistance to thinning and a high apparent viscosity, and is not readily usable in the applications described above.        4. The apparent viscosity at ambient temperature is 50-900 centipoise (cps), preferably 50-300 cps at a shear rate of 200 rpm or less. With such a low resistance to thinning, the slurry is regarded as thin, preferably when the apparent viscosity is less than 100 cP. The achievement of such thin, high-solids slurries generally requires a viscosity modifier to facilitate the breakdown of gels with minimum agitation. The desirability of thin slurries is that they are easily handled, and are more amenable to the application of subsequent processing steps that deliver desirable properties such as sprayed coatings that have a high strength because the low viscosity delivers an ease of application and surface coverage with a low water content that would otherwise cause cracking and/or low strength when dried due to the excessively high permeability from water evaporation during drying. In many applications, this desirable property of thin slurries is augmented by the surface chemistry that arises from the use of high surface area oxide particles used to produce the slurry.        5. The stability of the slurry is such that it can be used up to many months after manufacture: The characterization of slurry stability is often somewhat arbitrary and may involve meeting a number of criteria. For example:                    (i) One criterion may be that of pourability/flowability, so that more than 80% by weight, preferably greater than 90% by weight, of a bulk sample in, say, a 1-m3 container pours off after 7 days of undisturbed gravity settling;            (ii) Another criterion may be that after 7 days of undisturbed (unagitated) gravity settling, water separation, called syneresis, in a vessel of 1 m3 container of 1 meter depth is less than 30 millimeters; or, in 30 days, the syneresis is less than 50 millimeters.            (iii) Another criterion may be that the slurry has less than 1% sediment (“heel”) after 30 days.            (iv) Another criterion may be that the syneresis is less than 5% after 30 days, or preferably 3%.                        
It is recognized that these thin, high-solids slurries of variable chemical reactivity do not have a long intrinsic lifetime, and some degree of sedimentation occurs. For many applications, the re-suspendability of the slurry is much more critical than long-term slurry homogeneity, because agitation can be provided at the point of storage, and if required, such agitation during storage may be intermittent.
The choices of viscosity modifiers and stabilizers to produce stable, thin, high-solids slurries are generally associated with the surface charges on the particles and the ionic strength of the water. The viscosity modifiers and stabilizers are generally not specific to the method of manufacture of the slurry.
The prior art for the production of hydrated lime slurries from kiln lime is well understood. Of relevance to this disclosure, U.S. Pat. No. 3,573,002 discloses a two-stage process, and uses steam pressure to overcome differences in the hydration rates of lime and magnesia that otherwise cause significant issues in making slurries of mixed alkaline earth hydroxides. The use of pressure vessels adds to the complexity and cost of a slurry plant. The kiln lime is generally a burned lime in which the granules are sintered to give a moderate surface area. This is acceptable because the hydration process of lime causes the granules to break up from the stresses induced as the particles expand during the reaction to accommodate the water. There is a need for a process that can produce hydroxide slurries from un-sintered materials without the need for high-pressure processing. Conventional lime hydrator plants, operating at ambient pressure, require inputs of lime with low magnesium oxide content.
The prior art for the production of magnesium hydroxide slurries that meet the industrial requirements listed above is characterized by the initial solids materials used to make the slurry. It is noted that these processes do not generally use the approach of using high-pressure steam, as disclosed for hydrated lime with magnesia, to accelerate the magnesia hydration reactions. The magnesia particles generally do not exhibit significant fracturing from the hydration processes. These classes of materials for magnesium oxide slurries are:
a) Precipitated Magnesium Hydroxide (PMH).
PMH is generally produced by the precipitation of magnesium hydroxide from brines by the addition of hydrated lime. The prior art, described below, focuses on the use of (i) viscosity modifiers that thin the slurry and (ii) stabilizers that facilitate the stability of the slurry, formed by agitating (deflocculating) the washed precipitate in water. The specific viscosity modifiers are selected to deal with the presence of significant amounts of residual chloride ions in the washed precipitate. PMH does not require grinding or hydration to make the slurry because the particle size of the PMH is similar to that of the desirable slurry, namely about 25 microns or less.
Specifically, Japanese Patent No. 54150395 describes the production of slurry by grinding dried magnesium hydroxide to a specified particle size and then mixing with water under agitation. U.S. Pat. Nos. 5,762,901 and 5,514,357 describe the stabilization of slurries in which the slurry contains chloride ions in 0.30-0.42%, by weight on an MgO basis. These describe the use of a cationic polymer and, if required, a thickening agent to stabilize the slurry, so that the slurry formed by physical deflocculation is stable, so that it can be transported and stored without substantial agglomeration of the magnesium hydroxide solids. U.S. Pat. No. 5,877,247 describes the stabilization of slurries formed from solid magnesium hydroxide using a combination of one or more polymeric dispersants and one or more water-soluble alkali metal salts. Patent EP 1009717 A4 discloses the production of stabilized magnesium hydroxide slurry using wet milling of magnesium hydroxide granules to give control of the median particle size, controlled particle size range, and controlled surface area of the Mg(OH)2 solids in the slurry.
The addition of viscosity-modifying agents and dispersants to slurries to control viscosity, stability and dispersability, is well-established art. Such viscosity-modifying agents or dispersants can include decomposable phosphates (FR 2399485); carboxylic acid type polymeric surfactants (JP 5-208810); polyanion and anion of strong acids such as HCl, H2SO3 or H2SO4 (DE 4302539); polymeric anion dispersant and water-soluble alkali metal salt (AU 48785/93); sulphomethylated acrylamide homopolymers or copolymers (U.S. Pat. No. 4,743,396); alkaline salts of a sulfosuccinic ester product (DE 3323730); alkali metal silicate and hydroxide and/or mineral acid salts (J 62007439); organic or inorganic dispersants (J 61270214); xanthan gum and lignin sulphonates (CA 110 (10):7837e); carboxymethylcellulose (CA 104 (6):39729k); cationic polymers (U.S. Pat. No. 4,430,248); ferrous hydroxide or aluminum hydroxide (CA 79(8):44013S) and polyacrylates (U.S. Pat. No. 4,230,610).
b) Dead-Burned Magnesia (DBM).
DBM, generally in granules of about 25 mm or less, is generally produced by the calcination of the mineral magnesite. DBM is sintered, with a very low specific surface area, often below 0.1 m2/gm. When mixed with water, the hydration of DBM to produce magnesium hydroxide is very slow, over many days and weeks. The prior art for slurries formed from DBM is focused on activating the hydration process, dealing with the propensity of insoluble magnesium hydroxide to coat the small surface area presented, and to slow down the reaction. The means of activation include wet milling to regenerate the surface, and preferably in hot water to take advantage of the fact that the hydration reaction is thermally activated, and the use of chemical additives that are associated with lifting the coating from the surface. Generally, viscosity modifiers and stabilizers are used to produce a thin, stable high-solids slurry, in the same manner required for (a).
Specifically, U.S. Pat. No. 5,487,879 A describes the process of production of a stabilized, pressure-hydrated magnesium hydroxide slurry from ground DBM. A mixture comprising ground DBM and water is pressure hydrated to provide a pressure-hydrated slurry. The pressure-hydrated slurry is then de-agglomerated. If desired, chloride ions and cationic polymer can be added to further stabilize the slurry. The pressure is preferably 2-7 bar and the temperature is preferably that of wet steam at that pressure. The process is catalyzed by the introduction of chloride ions. This patent teaches the use of magnesium chloride to catalyze the hydration of the DBM.
As an alternative to the pressure process, the wet milling of the DBM granules is described in the prior art. U.S. Pat. No. 5,906,804 A and European Patent No. 0772570 B1 describe a process for producing a stable magnesium hydroxide slurry in which wet grinding calcined magnesia granules having a particle size of about 25 mm or less, and hydrating the finely divided magnesia in a hydration zone, wherein the finely divided magnesia is mixed with water under agitation and heat so as to produce a magnesium hydroxide slurry having at least 80% hydration; and passing the slurry through a second particle reduction zone so as to produce slurry particles, wherein 90% of the slurry particles have a size less than 50 microns. A viscosity-modifying agent is added to ensure a maximum viscosity of 1000 cP. The final product is described as stable, pumpable, magnesium hydroxide slurries having a solids content of at least 40%. The viscosity-modifying agent is selected from the group consisting of either inorganic acids having a molecular weight less than 130 amu, or inorganic salts thereof having an alkali metal as a sole cation; or carboxylic acids having a molecular weight of less than 200 amu, optionally containing one or more hydroxyl groups and salts thereof, excluding salts having alkali metal as a sole cation; or polyhydric alcohols and carbohydrates containing two or more hydroxyl groups and having a molecular weight of less than 500 amu; or alkaline earth oxides, hydroxides and a combination thereof. This patent teaches recirculating the parent (unhydrated solids) through a loop until the particle is substantially consumed.
Generally, viscosity modifiers and stabilizers are used to produce a thin, stable, high-solids slurry from DBM, in the same manner required for (a).
c) Granular Caustic Calcined Magnesia (GCCM).
GCCM is also produced from the calcination of the magnesite. GCCM granules may be extracted from a kiln at an earlier stage of process than DBM. The surface area of GCCM is typically in the range of 25-60 m2/gm. GCCM is also sintered, but to a lesser degree than DBM.
In the case of GCCM, the rate-limiting process for slurry formation is the wet milling of the granules. The faster hydration is associated with the use of shorter time milling processes compared to DBM. In common with slurries made from PMH and DBM, high-solids slurries require viscosity modifiers and stabilizers to produce a thin, stable, high-solids slurry.
Specifically, the production of slurries from GCCM is described in Japanese Patent No. 5-279017 and Japanese Patent No. 5-279018. GCCM is introduced into a hydration tank equipped with a stirrer or agitator and is simultaneously milled by steel balls or other form of abrading apparatus. Bron et al., Chemical Abstracts (CA) 68(2): 5884e (1966), refers to the hydration of magnesite-derived MgO. In this case, magnesium hydroxide was produced during boiling or short wet grinding of the MgO with water in a ball mill. European Patent No. 0599085 describes a process in which GCCM is comminuted in the wet state with a wet pulverizer and hydrated in the presence of an alkaline aqueous medium that included sodium hydroxide at an elevated temperature of not less than 70° C. The resultant pulverized material is classified into fine and coarse particles using a classifying means that is generally set to restrict the passage of particles in excess of 20 microns. Subsequently, the coarse particles are recycled to the wet pulverizer. By subjecting GCCM to concurrent wet pulverization and hydration in the presence of a heated alkaline aqueous medium, magnesia can be simultaneously comminuted and hydrated under rapid heating to produce an active magnesium hydroxide showing a low viscosity, even at a high concentration.
South Korean Patent No. 9301256 describes formation of active magnesium hydrate made from light-burned magnesite that is subjected to wet crushing with water, an alkali stabilizer inclusive of sodium hydroxide, and dispersing agent inclusive of polycarboxylate using reaction heat and crushing heat.
Generally, viscosity modifiers and stabilizers are used to produce a thin, stable, high-solids slurry from GCCM, in the same manner required for (a) and (b).
Powdered Caustic Calcined Magnesia (PCCM).
PCCM may be produced by simply grinding GCCM, or may be directly produced by the flash calcination of ground magnesite powders, or by drying slurries produced by any of the aforementioned processes and flash calcining the dried hydroxide. Most flash calciners, however, generally have the undesirable property that some particles are exposed to very high temperatures from the hot combustion gas, and calcine and sinter quickly, so that the product has variable specific surface area, and variable hydration properties. The average properties of flash calcined PCCM are otherwise similar to those of PCCM from grinding granules. Indirect heating, counterflow calciners, as described by Sceats and Horley, for example, in WO 2007/112496 (incorporated herein by reference), produce uniformly calcined PCCM with minimal sintering and a high specific surface area, which can be in the range of 100-250 m2/gm with the degree of calcination of 90-98%. Such calcined PCCM has markedly different surface chemical properties than PCCM produced by conventional methods.
The production of slurries from PCCM has been previously described. JP-2-48414 refers to a process of producing slurry from PCCM having a solids content of 5-70% wt % at above 50° C. under agitation, wherein some slurry is periodically removed and replaced by hot water and magnesia to obtain a uniform slurry density. JP 3-252311 refers to a process for preparing PCCM grinding the GCCM to a mean particle size of 5-10 microns and then subjecting the ultra-fine powder in an acidic reaction. JP-01-212214 refers to a method of manufacturing a PCCM slurry having 10-50% wt % Mg(OH)2, wherein magnesia having a mean particle diameter of less than 100 microns is hydrated in the presence of alkali metal ions and/or alkaline earth metal ions and also in the presence of the hydroxide ion, nitrate ion, carbonate ion, chloride ion and/or sulphate ion. DD 272288 describes hydration of MgO resulting from MgCl2 thermal decomposition carried out by (a) pre-hydrating MgO in one or more series or parallel connected hydration reactors; and (b) grinding in one or more series or parallel connected hydration reactors. JP 03-60774 refers to the production of magnesium hydroxide slurries that includes the step of slaking finely pulverized light burnt magnesia that is obtained by firing naturally produced magnesite with water with heating to 85-100° C. Sodium hydroxide is added as a hydration accelerator. It is known from JP 5-208810 and JP 3-252311, for example, that magnesia may be produced by calcination of magnesite followed by particle reduction to obtain ultra-fine particles having a mean particle size of 5-10 microns that are then hydrated to form magnesium hydroxide slurry. The hydration process can be carried out in a particle reduction zone.
It is also known to use additives to accelerate the hydration of MgO to Mg(OH)2 and/or to modify the crystal shape of the magnesium hydroxide product during hydration. Such additives include citric acid or magnesium chloride (see CA 110(24):215623f), short chain carboxylic acids or corresponding salts such as magnesium acetate (JP 3-197315, JP 01-131022 and DD 280745), ammonium chloride (DD 241247); magnesium chloride, magnesium acetate, magnesium sulphate or magnesium nitrate (DD 246971); inorganic or organic acids such as HCl or acetic acid or their magnesium salts such as magnesium chloride, or magnesium acetate (CA111(18):159019n), proprionic acid (JP 63-277510), n-butyric acid (JP 63-277511), and sodium hydroxide (JP 03-60774).
JP-3-197315 refers to the production of a magnesium hydroxide slurry having 3-70 wt % and more preferably 20-50 wt % solids as an intermediate in the production of magnesium hydroxide crystals having hexagonal plate-like crystals, which are obtained as a final product of the hydration of magnesia. These crystals are utilized as a fire retardant. JP-1-131022, which is discussed in the prior art preamble of JP-3-197315, states that the purpose of addition of magnesium salts, such as magnesium acetate, or organic acids, such as acetic acid, is for controlling the rate of hydration or for controlling the growth of magnesium hydroxide crystals. The crystals that are obtained by the hydration process of this reference are regular in shape, thereby avoiding the formation of agglomerates.
Generally, viscosity modifiers and stabilizers are used to produce a thin, stable, high-solids slurry from PCCM, in the same manner required for (a), (b) and (c).
There is a need to produce low-emissions intensity slurry products to mitigate the impact of global warming. The production of PMH from brines is energy intensive and uses hydrated lime that is generally produced in lime kilns that have significant CO2 emissions from both the energy consumed and from the calcination of limestone. The production of DBM and GCCM use energy-intensive kilns, that also have high CO2 emissions from both the energy consumed as well as from the calcination of magnesite, as does the production of PCCM from traditional flash calciners.
In contrast, the production of PCCM using calciners, of the type described by Sceats and Horley, with indirect heating of magnesite entrained in steam produces, after steam condensation, a pure CO2 stream that can be liquefied and sequestered, thereby significantly reducing the carbon footprint. Indirect heating, with counterflow, is energy efficient and, for the purpose of this specification, produces a PCCM with a very high surface area, in the range of 100-200 m2/gm. In addition, this type of calciner also produces high surface area lime CaO, dolime CaO.MgO and semidolime MgO.CaCO3. More generally, all carbonate minerals are a mixture of limestone, dolomite and magnesite, and the calcined material from this reactor is a powder mixture of the oxides and unreacted carbonates. These powdered caustic calcined carbonate powders are the feedstock to produce the hydroxide slurries described in this disclosure. The high surface area calcined carbonate powders are very reactive, and there is a need for a manufacturing process that can use such feedstocks for the production of slurries. In the development of this process for such high surface area materials, the process described herein can also be applied to the formation of slurries from caustic calcined carbonate powders, such as PCCM, produced using traditional flash calciners or by grinding granular caustic calcined carbonate materials, such as GCCM. The prior art described above for the production of slurries from PCCM cannot be used for the very reactive powders. There is a need for a slurry production process for powdered caustic calcined carbonates that can be applied generally to powdered caustic calcined feedstock produced by any means, and of any composition. The description below is based on PCCM because magnesia is the most difficult calcined carbonate material to slurry. The disclosure is equally applied to any powdered calcined earth carbonate, including mixtures.
Alternatively, high surface area PCCM can be obtained by drying a magnesium hydroxide slurry formed by any of the aforementioned processes, and flash calcining this material at lower temperatures, preferably below 600° C. to rapidly dehydroxylate the hydroxides to reform PCCM. The lower temperature of dehydroxylation compared to decarbonation means that the sintering of the PCCM produced by the calcination of hydroxide particles is significantly reduced, so that the surface area is further increased. The calciners described by Sceats and Horley produce a very high surface area PCCM, of about 250 m2/gm, from a dried hydroxide feed, from an initial PCCM having a surface area of less than 200 m2/gm. In this approach, the surface area of the PCCM is highest when the slurry has been produced by hydrating a high surface area PCCM. As described above, the surface particles in slurries produced from higher surface area PCCM are characterized by higher concentrations of reactive species such as peroxide and superoxide and, in many applications, these species are beneficial. Repeating the steps of dehydration, flash calcination and hydration allows an incremental increase in the surface area of the PCCM produced in each step, and thus the concentration of reactive species. The hydration process in each of these hydration steps requires the use of the slurry production process described in this disclosure because the rate of heat release becomes too fast for conventional slurry production processes.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.